Even though this sounds complicated, and perhaps unnecessary, I feel these can be easily accomplished.
The splitting into three would be an on death event which spawns three smaller wither storms with lower stats and they would be different entities.
Similar to bane of arthropods and smite, you could make another entity subclass or whatever you call those just for the storm and the mini storms.

But only the modder can decide whether or not to add it in the end. Let's wait and see.

This mod SUCKS. The player models are ugly, Soren looks like a bald gandalf, And the witherstorm looks like a witherstorm that took a shower. Learn progamming and ill be happy. Kiriot22 have the better mod.

I have a great idea for the mod you should add a recipe to create a command block where it is like the command block in minecraft because it will be but it should have a setting where you it says add in code for wither storm but you need a crafting recipe for that. And there should be a new ore block called a command ore 10% of finding it and there should have control panels for the command block and white clay in the corners.